How can I update all Snap packages?

sudo snap refresh 

Will do this. It is part of snapd 2.0.8, which landed 2016-06-13 in xenial-updates.

snap refresh --list

Only lists the updates without refreshing the packages.

snap info <snap name>

Can show which versions are available for a particular package.

According to Snap tutorial Snaps are automatically updated in the background once per day.

Generally, you don't need to do anything.

snapd, in Ubuntu installs, will automatically check for updates. That's a key feature of snaps.

Here's how to determine how often snapd will automatically check for updates: (source)

$ snap refresh --time
timer: 00:00~24:00/4        <------ "/4" means refresh every 4 hours.
last: today at 17:15 CDT
next: today at 21:45 CDT